Process for detaching layers of material

ABSTRACT

A process for detaching two layers of material according to a weakened zone defined between the layers. This process includes the thermal annealing of a structure that incorporates the layers, with the annealing bringing the temperature from a starting temperature to a final annealing temperature while evolving according to a first phase up to a transition temperature, then according to a second phase during which the rise in temperature per unit of time is greater than that of the first phase. The invention also concerns an application for using this process in a particular semiconductor fabrication technique.

BACKGROUND ART

The present invention generally relates to the processing of materials,and more particularly to substrates for electronics, optics oroptoelectronics.

More precisely, the invention relates to a process for detaching twolayers of material according to a weakened zone defined between the twolayers. This process comprises thermal annealing of a structure of thelayers, wherein the annealing brings the temperature from a startingannealing temperature to a final annealing temperature in a controlledand defined manner.

Processes of the type mentioned above are already known in general. Inparticular it is known to utilize these processes for detaching twolayers of material coming from the same substrate, between which aweakened zone has been previously defined by implantation of species inthe substrate. The implanted species can be ions or atoms. It is thusknown to implant a substrate of a semiconductor material such as siliconwith a species such as hydrogen or helium to obtain this weakened zone.

The weakened zone is determined as a function of the nature of thematerial, the nature of the implanted species and the implantationenergy (this weakened zone typically being a plane or zone orientedparallel to the implantation face of the substrate).

It is also possible to produce the weakened zone by other meansgenerally known per se, for example by constructing an intermediateregion of a porous material between two regions of dense material, byconstituting a layer of oxide embedded in a substrate (for example asubstrate of silicon-on-insulator or SOI type), or even by bondingtogether two layers, with the bonding interface serving as the weakenedzone.

The detachment of two layers at the weakened zone can be used to createthin layers (whose thickness can be between a fraction of a micron andseveral microns), as is described for example in U.S. Pat. No.5,374,564.

This document describes a process known under the generic name ofSMART-CUT®, the aim of which is to manufacture structures of the SiliconOn Insulator (“SOI”) type. The main steps in this process are thefollowing:

formation of an oxide layer in a so-called upper plate of silicon, wherethe oxide layer corresponds to an embedded oxide layer of a SOIstructure,

ionic implantation of hydrogen or other ions in this upper plate so asto create the weakened zone, and to delimit the SOI structure by thiszone on the one hand (i.e., the layer situated on one side of theweakened zone which includes the embedded oxide layer), while defining asilicon heel or reusable silicon substrate on the other side of theweakened zone,

adhering, preferably by molecular bonding, the upper plate onto asupport plate called a stiffener which can be made of silicon or anothersemiconductor material, and

heating the structure with a view to detach and obtain an SOI structurecomprising, as one layer, the stiffener or support plate, the embeddedoxide layer and the layer of silicon located between the embedded oxideand the weakened zone, while also providing, as the other layer, thesilicon heel located on the other side of the weakened zone, which heelcan be re-used in further operations, e.g., to transfer further layersof silicon to other support substrates or stiffeners.

In this respect, the heating can be one that provides completedetachment, with the SOI and the heel effectively exiting from theannealing oven detached and as separate components, or if the thermalbudget is not sufficient to complete the annealing, to aid only ineffecting a separation between the SOI and the heel due to the formationof microcavities, microbubbles or microcracks in the weakened zone. Theseparation corresponds to a condition or state preceding detachment inwhich the SOI and the heel are still attached by Van der Waals forcetype bonds, or again by a simple suction effect of the two parts to bedetached.

In this case, detachment is finalized and completed after the initialheating, for example, by introducing mechanical energy (such as theintroduction of a projecting element such as a blade at the level of theweakened zone, etc.), or by further heating of the structure.

A complementary processing such as polishing is then conducted to reducethe roughness of the surface of the SOI which originates from splittingand detachment. It is usual to find roughness specifications notexceeding 5 Angstroms in root mean square (“rms”) value. The measures ofroughness are generally made by an atomic force microscope (“AFM”). Withthis type of instrument, roughness is measured on surfaces swept by thepoints of the AFM, ranging from 1×1 μm² to 10×10 μm² and less frequently50×50 μm², or even 100×100 μm². It is also possible to measure thesurface roughness by other methods, in particular by the bias of a hazemeasurement.

This method has the particular advantage of enabling rapidcharacterization of the uniformity of roughness on the entire surface.This haze, measured in ppm, originates from a method utilizing theproperties of optical reflectivity of the surface to be characterized,and corresponds to an optical base noise diffused by the surface, interms of its microroughness. An example of a graph illustrating therelationship between the haze and the roughness of this surface for anSOI surface is shown in FIG. 1.

The SMART-CUT® process can also be used to constitute structures otherthan SOI, for example Silicon On Anything (“SOA), or even Anything OnAnything (“AOA”), i.e., any material on any other material which is thesame or different.

The known processes of detachment by heating generally utilize anannealing step for inducing the structure comprising the two layers todetach. This step starts at a relatively low initial heatingtemperature, which may be for example on the order of 350° C., to ahigher detachment temperature though not exceeding a value on the orderof 500° C., with a temperature evolution in the annealing oven increasedin a substantially constant manner at a rate on the order of 10° C. perminute.

The detachment temperature for these detachment annealings of the priorart corresponds to a final annealing temperature. Therefore, the knowndetachment annealings are performed with temperature evolution accordingto a substantially constant temperature ramp up, with the gradient ofthis ramp being on the order of 10° C./min. But it is often noticed thatthe surfaces originating from detachment (that is, the surfaces of thetwo layers opposite both sides of the weakened zone after detachment)exhibit a relatively pronounced degree of roughness, thus requiringsignificant additional processing to be conducted in order to attain thedesired surface state for further use of the components. For example, inthe event of detachment of the layers of a plate of material such assilicon for constituting a SOI, completion of detachment generallyresults in roughness of the order of 80 Angstroms rms (in AFM measure ona field of 10*10 microns).

In order to further process the detached material, as well as to reusethe wafer from which the layer has been detached, there is a need forimproving the surface roughness of these components while minimizing orwithout having to perform the additional processing steps.

In addition, with the known detachment annealing processes, degradationof the peripheral potions of the structures to be detached is alsoobserved. FIG. 2 illustrates the typical result of microscopicobservation of the peripheral edge of the surface of an SOI after itsdetachment by annealing according to the prior art, with the peripheralregion being called a crown or rim. This drawing shows numerousstructural irregularities in the SOI crown. This figure illustratesdegradation of the SOI crown, with this degradation being consequent tothe detachment annealing and being applicable to structures other thanSOI. Accordingly, it also would be beneficial to prevent, or at leastreduce, such degradation noticed as a consequence of annealingdetachment.

These disadvantages of the prior art are now remedied by the presentinvention.

SUMMARY OF THE INVENTION

The invention now allows one to perform detachment annealings forproducing an improved state surface of the surfaces resulting fromdetachment. As a function of the above, the invention is moreparticularly directed to reducing the roughness of the surfacesresulting from detachment, relative to known processes.

These advantages are obtained in a process for detaching of two layersof material along a weakened zone defined between the layers. Theprocess comprises thermal annealing of a structure that includes the twolayers by conducting a first and second annealing phases that areseparated by a transition temperature, and raising the temperature inthe second annealing phase at a rate that is greater than that of thefirst annealing phase so as to provide an improvement in surfaceroughness of detached surfaces compared to those obtained by anannealing process where the temperature is uniformly raised.

Preferably, the temperature is raised in the second annealing phase at arate that is at least 20 to 80% and more preferably at least 33 to 67%greater than that of the first annealing phase. Advantageously, thetemperature rises in the first annealing phase uniformly and at arelatively slow average gradient of no greater than about 10° C./min,while the temperature rises in the second annealing phase at arelatively rapid average gradient of at least about 15° C./min orgreater.

In a preferred embodiment, the material of the layers comprises asemiconductor material, and the transition temperature corresponds to atemperature that is just below that which causes detachment of thelayers.

The weakened zone is advantageously created by the implanting of ions orthe provision of a porous layer. When the two layers are part of asilicon on insulator structure, they can be provided by: forming anoxide layer in an upper plate of silicon, implanting ions in the upperplate to create the weakened zone which delimits the silicon oninsulator structure and a silicon heel, and bonding the upper plate ontoa stiffener, so that the thermal annealing detaches along the weakenedzone, as one layer, a silicon on insulator structure comprising thestiffener, the embedded oxide layer and a layer of silicon locatedbetween the embedded oxide layer, and, as the other layer, a siliconheel for reuse in further operations.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Other aspects, aims and advantages of the invention will emerge clearlyfrom reading the following description of an embodiment of theinvention, with reference to the accompanying diagrams in which:

FIG. 1 is a graph illustrating the relationship between the haze and theroughness of the surface which is obtained after detachment in the caseof an SOI surface,

FIG. 2 illustrates the typical result of microscopic observation of theperipheral edge of the surface of an SOI after its detachment byannealing according to the prior art,

FIG. 3 diagrammatically illustrates an annealing oven used to carry outthe invention,

FIG. 4a illustrates the spatial distribution of haze on the surface ofan SOI having undergone detachment annealing according to the prior art,

FIG. 4b is a graph representing overall the distribution of haze on thesurface of the SOI for the surface of which FIG. 4a illustrates thespatial distribution of haze, with the graph of FIG. 4b specificallyidentifying the average haze of the surface in question,

FIG. 5 graphically represents the evolution of temperature duringdetachment annealing carried out according to the prior art, and thesame type of evolution for two detachment annealings undertakenaccording to the present invention,

FIG. 6 illustrates highly diagrammatically the bonding of a layer ofoxidized and implanted silicon, and a stiffener, in a SMART-CUT® typeprocess,

FIGS. 7a, 8 a and 9 a visually represent the spatial distribution of thehaze on the surface of three SOIs following their detachment byannealing, the SOI of FIG. 7a having undergone standard detachmentannealing, the two SOIs of FIGS. 8a and 9 a having undergone detachmentannealings according to the present invention carried out according todifferent modalities,

FIGS. 7b, 8 b and 9 b are graphs similar to those of FIG. 4b overallshowing the distribution of haze for the SOI and respective of FIGS. 7a,8 a and 9 a (FIG. 7b corresponding as does FIG. 4b to SOI annealingaccording to a known technique),

FIGS. 10 to 12 are three similar representations of observation bymicroscope of the peripheral edge of three respective SOIs which haveundergone detachment annealing,

FIG. 10 corresponding to a SOI having undergone standard annealing(similar to FIG. 2), and

FIGS. 11 and 12 correspond to the respective SOIs of FIGS. (8 a, 8 b)and (9 a, 9 b).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention proposes a process of detaching two layers of materialaccording to a weakened zone defined between the two layers. Thisprocess comprises thermal annealing of a structure 10 comprising thelayers, with the annealing bringing the temperature from a startingannealing temperature to a final annealing temperature, wherein duringthe thermal annealing the annealing temperature evolves according to afirst annealing phase to a transition temperature, then according to asecond annealing phase during which the rise in temperature per unit oftime is greater than that of the first phase.

In the case of structures such as multilayer substrates comprising, asabove, SOI, SOA, or even AOA and a layer of a heel of re-usable material(silicon in the case of SOI), the shape of the structure is generallythat of a very fine crust designated by the term wafer.

Generally, the first phase is an initiation phase of detachment, and thesecond phase is a surface-finishing phase, and the second phase isfollowed by a heating stage at a substantially constant temperature,which corresponds to the final annealing temperature.

According to another aspect, the invention also proposes application ofa process according to one of the abovementioned aspects to detach twolayers of material delimited by a weakened zone, wherein the weakenedzone is produced within the scope of a process of the SMART-CUT® type.

A preferred material for the two layers is silicon, and the two layersof material can be two different layers of silicon, with onecorresponding to an SOI, and the other layer corresponding to a siliconheel.

The wafers to be treated are generally arranged vertically in the oven,as shown in FIG. 3, with the wafers to be split being designated byreference numeral 10, the oven being designated by reference numeral 20.

This vertical arrangement of the wafers aims to prevent the risks of thetwo detached layers of each wafer shifting, relative to one another, atthe end of detachment in the oven or in particular during subsequenthandling operations when the detached wafers are removed from the oven.

As the specifications of the surface state are, in some cases, veryspecific (in particular for a SOI), it is necessary to try to preventany relative shifting of the two detached layers so as to avoid therisks of any scratching on the surfaces of the two detached layers.

It is also noted, as illustrated in FIG. 4a, that the surface roughnessof a SOI 11 resulting from detachment annealing exhibits dissymmetry. Infact, this figure shows a zone of the SOI surface (situated between 9and 10 o'clock) in which the haze, and consequently the roughness, areincreased.

Such divergence results from the presence of hot points in the annealingoven. In the case of an oven such as illustrated in FIG. 3, it is moreprecisely about the translation of the vertical temperature gradient.Generally, the wafers to be detached are placed in the oven at anorientation of 90° relative to the representation of FIG. 4a.

With reference now to FIG. 4b, this shows the overall distribution ofhaze on the surface of the SOI of FIG. 4a.

It will be recalled that this SOI has undergone detachment annealingaccording to a process known in the prior art. FIG. 4b illustrates thatthe value of the average haze on the whole SOI is of the order of 87ppm. This value is directly associated with an average roughness valueof this SOI, and thus represents a reference value for measuring SOIroughness, characterized by the haze measurement.

This haze measurement, as well as similar measurements to be specifiedhereinbelow, are made according to the same protocol and using the sameinstrument, in this case by an instrument of the KLA TENCOR SURFSCANSPI™ type.

With reference now to FIG. 5, the evolution of the temperatureprevailing in the annealing oven, during three detachment annealings, asa function of time, has been diagrammatically illustrated. Moreprecisely, this figure comprises three curves 51, 52, 53 whichcorrespond to three different types of annealing.

The start of the temperature evolution during these three annealings isidentical, comprising an entry stage at 350° C., which corresponds tothe annealing entry temperature, followed by a rising ramp of atemperature having a gradient of the order of 10° C. per minute or less,as is generally from the prior art.

In the right part of the figure it is noticed that over time thetemperatures of the three annealings evolve differently. Moreparticularly, the curve 51 illustrates the evolution of a detachmentannealing according to the prior art.

In such annealing, after having been maintained at the entry level of350° C. for a first stage the temperature evolves according to the knowngradient ramp of the order of 10° C. per minute, up to a detachmentannealing temperature and final temperature of the order of 500° C.

It is nevertheless specified that the so-called detachment starttemperature, from which the beginning of detachment can be observed, mayhave a value of the order of 430° C. to 450° C.

The entry annealing temperature may have a value of less than 350° C.This will also be the case for annealings according to the presentinvention, illustrated by the curves 52 and 53.

The end of this annealing comprises a second stage, at this finalannealing temperature.

In the case of this known annealing, the temperature therefore evolvesaccording to a ramp.

The curve 52 represents the temperature evolution of annealing carriedout according to the present invention, in a first embodiment.

It is to be observed in this case that after having followed a part ofthe standard ramp having a gradient of the order of 10° per minute orless, once the temperature has been brought to a transition value of theorder of 430° C. to 450° C., temperature evolves according to a secondramp whereof the gradient is different to that of the first ramp.

The two ramps are illustrated as substantially linear. More precisely,the second ramp has a gradient substantially greater than that of thefirst ramp, the gradient of this second ramp being of the order of 16°C. per minute.

The difference in heating rate between the phases can range from atleast about 20% to much greater proportions. An upper limit of anincrease of about 80% is most often acceptable, although for certainmaterials other increases may be desirable. Generally, the heating ratein the second phase is about 33% to 67% and most preferably around 45%to 60% greater than that of the first phase.

Furthermore, for the annealing phases of the present invention, as wellas for the preferred annealing phases represented by the curve 53, thetwo linear ramps can be replaced by two non-linear ramps correspondingto the same general evolution in two phases detached by a transitiontemperature, as long as the second phase corresponds to a more rapidrise in temperature or heating rate than that of the first phase.

The annealing ends with a stage at the final annealing temperature ofthe order of 600° C.

The curve 53 illustrates a variation of the invention, in which thesecond ramp again has a gradient of the order of 16° per minute after atransition to 430° C. to 450° C.

In this instance, the second ramp continues up to a final annealingtemperature at an even greater value of the order of 800° C.

Accordingly, the detachment annealings according to the invention whichcorrespond to curves 52 and 53 are performed with a rise in temperaturecomprising two phases having different respective average ramps:

a “slow” first phase, whose average ramp can have a relatively lowaverage gradient of the order of 10° C./mm or less,

followed by a “rapid” second phase, having an increased averagegradient, such as on the order of 15° C./min or more, this second phasebeing intended to finalize detachment and to end, as will be explained,with an improved surface profile as well.

As noted above, the first and second heating rates or rises intemperature can be linear or not depending upon the specific materialused, the size of the material, the intended method for obtainingdetachment and the desired finish of the separated surfaces.

The first phase generally corresponds to classic annealing detachmentramp. The aim of this first phase, in the case of the invention, is tosupply the wafer to be detached with thermal energy leading almost todetachment.

More precisely, the first phase ends at a transition temperature thatcorresponds to one that causes onset of detachment following the thermalenergy supplied to the wafer during the first phase.

This onset of detachment is understood to mean the state in which a partof the weakened zone has effectively been separated, but that thisseparation has not spread though the entire weakened zone. This isaccordingly a state in which the wafer comprises a split “bubble” at thelevel of its weakened zone.

In conventional annealing, such as that shown by curve 51, thedetachment ramp continues beyond this detachment beginning along thesame average gradient to complete the detachment in the situation whereit is preferred to do so during the annealing.

In the case of the present invention, however, the temperature evolutiongradient is increased so as to continue annealing in a second phasecorresponding to a second ramp which is substantially greater than thegradient of the first phase.

It is specified that the transition temperature, which is, as we haveseen, the temperature corresponding to the onset of detachment, will beadapted in terms of the nature and dimensions of the wafer to bedetached; this adaptation could be made according to charts, orempirically.

In this respect, it should also be noted that the transition temperaturecorresponds more precisely to a thermal transition budget imparted tothe wafer, which in turn corresponds to a state of the wafer in whichonset of detachment takes place in the weakened zone of the wafer.

The aim of the second phase is to finalize detachment, by finishing upwith a surface state, in particular a surface roughness, that issubstantially improved relative to what is achieved from conventionalannealing methods.

In following the first phase, after a transition temperature such asthat defined and specified hereinabove, by a second phase ofsubstantially increased average temperature gradient, the result is asurface state of the detached layers which is substantially improvedrelative to what is achieved by the detachment annealing processes ofthe prior art.

It should also be pointed out that the second phase, apart from the factthat it has an increased average gradient, preferably also brings thewafer to a final annealing temperature that is likewise substantiallyincreased relative to the final temperature of annealing detachmentaccording to the prior art. Both of these are on the order of 500° C.,as illustrated by the curve 51. This is also illustrated by the finalannealing temperatures of the curves 52 and 53. And this characteristic,combined with the strong average gradient associated with the secondphase, actually results in a particularly advantageous surface state,the surface roughness of the detached layers decreasing further still.

To initiate detachment at a specific point on the weakened zone of thewafer, the divergence in temperature prevailing inside the oven can beexploited. For example, the presence of thermal gradients in an oveninside which the wafers are arranged vertically can be used toadvantage.

In the case where the wafer to be detached has been made up by bondingtwo layers having varying mechanical characteristics, the respectivemechanical behavior of the two bonded layers can also influencedetachment of the layers.

This is the case for example for SOIs prepared according to theSMART-CUT® process, where the SOIs are obtained by detachment at aweakened zone constituted by implantation, of the SOI and the siliconheel. In this regard, detachment does not mean the operation consistingof undoing the “bonding” i.e., a bond which had been created byadhesion.

Therefore, in the case of detachment of a wafer to constitute a SOI, abonding interface is created by bonding between the oxidized andimplanted plate and the stiffener, and detachment takes place at thelevel of the weakened zone created by implantation, this weakened zonebeing distinct from the bonding interface, even if it is in fact veryclose in position to the interface.

In this case, the two “layers” to be detached are therefore (1) both theSOI itself comprising the stiffener, the oxide and the fine slice orlayer of silicon that is to become a semiconductor substrate, and (2)the silicon heel.

Given the extreme fineness of the oxide and the fine layer of silicon ofthe SOI, the SOI has mechanical characteristics which are compatiblewith those of the stiffener, such that the two layers to be detachedhave the respective mechanical characteristics of silicon and of thestiffener, even if the stiffener is of a different material. Thestiffener is preferably made of silicon, but also can be any one of awide range of other materials, such as quartz.

From the point of view of the mechanical characteristics of the layers,each layer to be detached from the SOI can be assimilated respectivelyinto two layers, which were previously bonded together, to make up thewafer to be detached, i.e., the silicon layer and the stiffener.

The following applies to detachment of two layers which are compatiblein the same way into two layers previously bonded together, wherein eachof the layers to be detached is essentially composed of the material ofone of these two layers.

And with reference to the influence of the mechanical behavior of thelayers to be detached, the two layers to be detached from the SOI (thesilicon heel and the SOI itself) can thus have different mechanicalcharacteristics).

In addition, these two layers are obviously equivalent from themechanical point of view to a layer of silicon, and to the stiffener(these two elements having been assembled by bonding).

The two such bonded layers are not exactly flat, but there aretolerances on the surface evenness of such layers.

And bonding the two layers together (i.e., the oxidized and implantedsilicon S, and stiffener R, as shown in FIG. 6), was done so that theconcavities of the two respective layers face one another, as is showndiagrammatically in the figure. In this figure the concavities of thetwo bonded layers have been shown greatly exaggerated than in reality,and do not correspond in any way to a realistic scale.

During such bonding the concavities of the two layers are flattened soas to molecularly bond the surfaces of the two layers together. The twolayers of the resulting wafer (and consequently the two layers to bedetached, given that it was seen that mechanically these two layers wereequivalent to the two bonded layers) are thus slightly prestressedespecially in their central region.

These two layers will thus tend to move apart from one another as soonas detachment has been initiated in a movement of relaxing theconstraints due to the abovementioned molecular bonding. This benefitscontinuing detachment after the initial detachment described above andachieved on completion of the first phase of annealing.

This relaxation phenomenon becomes useful for detachment according tothe present invention of any wafer whereof the two layers to be detachedcan be assimilated into two slices which have previously been bondedtogether, by subjecting them to prestressing as they are adjusted to oneanother.

With reference to the process according to the present invention ingeneral, it has been said that the final annealing temperatures aregreater than the final annealing temperatures of the prior art. Moreprecisely, in a preferred variant of the invention beneficial resultswill be obtained with final annealing temperatures of the order of 500°C. to 800° C. And even more precisely, according to a preferred optionfor implementing the invention the final annealing temperature is of theorder of 600° C.

The gradient of the second ramp known as the rapid ramp is notnecessarily 16° per minute. This non-limiting value can be adapted topthe particulars of the situation and preferred values can be routinelydetermined by one of ordinary skill in the art. In any case, that valueit must be substantially greater than the value of the gradient oftemperature rise in the first ramp.

Similarly, the final annealing temperature, if effectively selected tobe substantially higher than for classic detachment annealings, is notlimited to the values described above which constitute preferred valuesonly.

Also, the profile of temperature rise in two rectangular rampsrepresented in the curves 52 and 53 constitutes only a particular modeof utilizing the invention.

In fact, a general characteristic of such annealing carried into effectaccording to the present invention is that it comprises a first phaseduring which the temperature is brought from a starting annealingtemperature (which can have a value of up to 350° C., as has beenmentioned) to a temperature corresponding to a starting detachmenttemperature for the wafer.

It is specified that the onset of detachment can for example be observedby following, using any means known in and of itself, the diameter ofgaseous bubbles generated by the detachment annealing from a weakenedzone defined by implantation, the gaseous bubbles originating from thecombination of micro-bubbles, micro-cavities or microcracks that areformed in the structure by the implanted ions.

This first phase corresponds to the first slow ramp of the curves 52 and53.

This is followed by a second phase during which not only is the rise intemperature continued, but even the growth in temperature is increasedper unit of time. This is different from the stabilized temperature thatis used in known detachment annealings, where the final annealingtemperature corresponds to a temperature which is only slightly greaterthan the starting detachment temperature.

Therefore, it is seen that the first phase corresponds to the mechanicaltriggering of detachment, while the second intensive annealing phaseallows not only finalizing of this detachment, but also obtaining a verygood surface state, this second phase corresponding to a finishingphase.

The starting detachment temperature, which corresponds to the transitiontemperature between the two annealing phases, can vary in terms of thecharacteristics of the structure to be detached.

In effect, the value of 430° C. to 450° C. mentioned above is notstrictly limiting: in terms of characteristics such as the material(s)of the structure, the dose of implantation energy utilized during thecreation of the weakened zone (when this zone has been created byimplantation), this temperature may vary to a certain extent.

For example, if the weakened zone is created by implantation, thenbefore the thermal budget is supplied to cause initial separation orbubbling, it has to be adapted when the implantation energy is modified.

FIGS. 7a to 9 a illustrate the differences in spatial distribution ofhaze on the surface of three SOIs which have undergone detachmentannealings according to three different modalities.

FIG. 7a thus illustrates distribution of haze on the surface of a SOIwhich has undergone classic detachment annealing, corresponding to thecurve 51 of FIG. 5.

This figure is to be compared to FIG. 7b, which, following the exampleof FIG. 4b, illustrates global distribution of haze on the surface ofthe SOI, and an average value of the order of 87 ppm.

FIG. 8a illustrates the spatial distribution of haze on the surface of aSOI which has undergone detachment annealing according to the invention,with a temperature evolution corresponding to the curve 52 of FIG. 5.

This figure is to be compared to FIG. 8b, from which an average hazevalue of the order of 73 ppm on the surface of the SOI is deduced.

A substantial decrease is noticed in the value of average haze and,consequently, of roughness on the surface of the SOI resulting fromdetachment.

FIGS. 9a and 9 b correspond to another SOI, having undergone detachmentannealing according to a temperature evolution according to the curve 53of FIG. 5.

In this case, as shown more particularly in FIG. 8b, the average valueof haze on the surface of the SOI is of the order of 5 ppm.

This corresponds to an extremely significant decrease in the resultantsurface roughness of the SOI compared to that obtained from knownannealing processes.

FIGS. 10 to 12 illustrate the evolution of the state of the crown ofthree respective SOIs which have undergone detachment annealingaccording to the three respective modalities described above(corresponding to the three respective curves 51, 52, 53 of FIG. 5).

These FIGS. 10 to 12 illustrate a significant decrease in the peripheraldegradation of the SOI structure, with the most degraded structurecorresponding to classic annealing, the best preserved structurecorresponding to annealing done according to the present invention andcorresponding to the curve 53 of FIG. 5.

It is specified that although the abovementioned examples have beendescribed in reference to detachment, thus generating a SOI, theinvention applies to detachment of any other structure comprisingweakened zone which effectively delimits the layers to be detached.

What is claimed is:
 1. A process for detaching of two layers of materialalong a weakened zone defined between the layers, which processcomprises thermal annealing of a structure that includes the two layersby conducting a first and second annealing phases that are separated bya transition temperature, and raising the temperature in the secondannealing phase at a rate that is greater than that of the firstannealing phase so as to provide an improvement in surface roughness ofdetached surfaces compared to those obtained by an annealing processwhere the temperature is uniformly raised.
 2. The process of claim 1,wherein the temperature is raised in the second annealing phase at arate that is at least 20 to 80% greater than that of the first annealingphase.
 3. The process of claim 2, wherein the temperature is raised inthe second annealing phase at a rate that is at least 33 to 67% greaterthan that of the first annealing phase.
 4. The process of claim 2,wherein the temperature rises in the first annealing phase uniformly ata relatively slow average gradient of no greater than about 10° C./min.5. The process of claim 2, wherein the temperature rises in the secondannealing phase at a relatively rapid average gradient of at least about15° C./min or greater.
 6. The process of claim 1, wherein the materialof the layers comprises a semiconductor material.
 7. The process ofclaim 6, wherein the transition temperature corresponds to a temperaturethat is just below that which causes detachment of the layers.
 8. Theprocess of claim 7, wherein the transition temperature is about 430° C.to 450° C.
 9. The process of claim 6, wherein the first annealing phasebegins at a temperature of no greater than about 350° C.
 10. The processof claim 6, wherein the second annealing phase ends at a temperature ofabout 500° C. to 800° C.
 11. The process of claim 10, wherein the finalannealing temperature is on the order of about 600° C.
 12. The processof claim 10, wherein the second annealing phase is followed by a heatingstage at a substantially constant temperature of about 500° C. to 800°C.
 13. The process of claim 12, wherein the substantially constanttemperature of the subsequent heating stage is 600° C.
 14. The processof claim 6, wherein the weakened zone is created by the implanting ofions or the provision of a porous layer.
 15. The process of claim 1,wherein the two layers are part of a silicon on insulator structure thatis provided by: forming an oxide layer in an upper plate of silicon,implanting ions in the upper plate to create the weakened zone whichdelimits the silicon on insulator structure and a silicon heel, andbonding the upper plate onto a stiffener, so that the thermal annealingdetaches along the weakened zone, as one layer, a silicon on insulatorstructure comprising the stiffener, the embedded oxide layer and a layerof silicon located between the embedded oxide layer, and, as the otherlayer, a silicon heel for re-use in further operations.